The invention relates to a method for making a rubber base composition containing silicic acid. This method is carried out in two stages in separate mixers and the composition ingredients silicic acid, silane and other usual additives except vulcanization ingredients are mixed and the silicic acid reacts with the silane.
Rubber compositions containing silicic acid are used today for making the most varied rubber products, especially, for making tires for motor vehicles. In motor vehicle tires, the silicic acid replaces for the most part the earlier conventional carbon black constituents. This is so because it has been shown that motor vehicle tires acquire improved tire characteristics with respect to wet resistance and rolling resistance because of the addition of silicic acid. For producing the rubber composition, a silicic-acid containing rubber base composition is produced first in a base mixing process in one or several mixing stages. This rubber base composition does not yet contain vulcanization ingredients (vulcanization chemicals such as sulfur, sulfur donors, accelerators or resins). In a final mixing process, the vulcanization ingredients (vulcanization chemicals) are added to the rubber base composition and the finished rubber composition (final composition) results.
In the production of the rubber base composition, coupling agents in the form of silanes are added to the rubber base composition in addition to the silicic acid in order to make possible a binding of the silicic acid to the rubber. The silanes are, inter alia, bifunctional organic silanes such as 3-mercaptopropyl triethoxy silane, 3-thiocyanato-propyl trimethoxy silane or 3,3xe2x80x2-bis(triethoxy silylpropyl)polysulfide with 2 to 8 sulfur atoms. In a first step, and during the base mixing, the added silane reacts with the silanol groups of the silicic acid and, in a second later step, during the vulcanization of the rubber composition, the silane reacts with the rubber, whereby the silicic acid is bonded to the rubber via the silane. The first reaction step and therefore the manufacture of the rubber base composition is very time intensive.
U.S. Pat. No. 5,804,636 discloses a method for making a silicic-acid containing rubber base composition of the kind mentioned initially herein. In a first stage of the method, the composition ingredients of rubber, silicic acid and additional additives except the vulcanization ingredients are mixed with each other at a temperature in the range of 165xc2x0 C. to 170xc2x0 C. in a first mixer. After mixing the composition ingredients, the composition is removed from the first mixer, cooled and transferred to a second mixer. In the second mixer, silane is added to the composition and the composition is mixed to completion at a temperature of 135xc2x0 C. and the silane reacts with the silicic acid. Because of the high temperature of the composition in the first mixer, it is ensured that the composition ingredients disperse rapidly and well; whereas, a thermal decomposition of the silane is avoided by maintaining the temperature of 135xc2x0 C. in the second mixer. Furthermore, by using two mixers, the advantage is provided that, at the same time, a rubber base composition without silane is mixed in the first mixer and, in the second mixer, a rubber base composition with silane can be mixed to the end. In this way, the expenditure of time for producing a rubber base composition containing silicic acid is shortened.
It is, however, noted that in the method set forth in U.S. Pat. No. 5,894,636, the silicic-acid containing rubber base composition must be cooled down in an intermediate step which leads to an additional expenditure of time and storage.
It is an object of the invention to provide a simple method for producing a silicic-acid containing rubber base composition which can be executed with a reduced expenditure of time.
The method of the invention is for making a silicic acid containing rubber base composition. The method includes the steps of: providing composition ingredients of rubber, silicic acid, silane and additional additives except for vulcanization ingredients; providing first and second mixers separate from each other wherein the ingredients except for the vulcanization ingredients can be mixed; introducing all of the composition ingredients into the first mixer except for the vulcanization ingredients simultaneously or at time intervals; mixing the composition ingredients introduced into the first mixer to a composition in the first mixer; transferring the composition formed in the first mixer directly into the second mixer without an intermediate storage thereof; and, mixing the transferred composition formed in the first mixer at least almost to completion in the second mixer with the temperature lying in the temperature range of 130xc2x0 C. to 180xc2x0 C. at least over the greatest part of the dwell time of the composition in the second mixer wherein the silicic acid reacts acceleratedly with the silane.
In the first and second mixer, a rubber base composition is therefore produced and, in the second mixer, the silane reacts almost completely with the silanol groups of the silicic acid (that is, the rubber base composition is then at least mixed almost to the end). At further processing stations, the rubber base composition is processed further and, in the last processing station, the vulcanization ingredients (vulcanization chemicals) are added to the rubber base composition and the rubber base composition is there mixed to completion in a final mixing process.
In the first mixer, the usual composition ingredients are dispersed in the rubber (or in the rubbers). Here, the temperature of the composition is increased up to a maximum of 180xc2x0 C. in order to substantially prevent a scorching of the composition. It is possible that the composition temperature lies below 130xc2x0 C. in advance of transferring the composition from the first mixer into the second mixer. In this case, the composition temperature is increased to a temperature in the temperature range of between 130xc2x0 C. and 180xc2x0 C. in the second mixer before the composition is mixed to the end in the second mixer. To adjust the temperatures in the mixers, the fill level of the mixture, the rpm of the rotors of the mixers, the mixer temperature control and the plunger pressure of the mixers (insofar that the corresponding mixer includes a plunger) are matched to each other.
An advantage of the invention is especially that the method can be executed with a reduced expenditure as to both time and space because an intermediate storage and cooling of the rubber base composition after separating the same from the first mixer is unnecessary. A further advantage of the invention is that all composition ingredients of the rubber base composition are introduced into the first mixer and therefore also only stationary metering devices are needed in the region of the first mixer. Furthermore, it is ensured that the individual composition ingredients in the rubber base composition are present in the correct amounts. (At this point, it is noted that in the method known from U.S. Pat. No. 5,804,636, the above is not ensured. Accordingly, it can happen that, when separating the rubber base composition from the first mixer, composition ingredients remain in the first mixer so that the rubber base mixture in the second mixer is supplied with too much silane; on the other hand, it can be that residual amounts from previous mixture operations are separated from the first mixer with the rubber base composition so that the rubber base composition is supplied with too little silane in the second mixer.)
Compared to a method for producing a rubber base composition wherein only a single mixer is used, the invention affords the advantage that the first and the second mixers can be operated simultaneously so that the clock time is shortened in which a rubber base composition is mixed to the end. A shortening of the clock time can also be achieved in that one sets up two individual mixers wherein a rubber base composition is formed in each of the mixers independently of each other. However, this would require much space and furthermore a metering device would be needed for each mixer or a movable metering device would be needed which can serve both mixers, which is complex and expensive.
According to another feature of the invention, the composition is heated in the first mixer to a temperature which lies in the temperature range of 110xc2x0 C. to 140xc2x0 C. The advantage of this embodiment is that, in the first mixer, already a high temperature is achieved and therefore a good dispersion of the composition ingredients is ensured. Furthermore, in this case, only a slight increase in temperature or no temperature increase need take place in the second mixer.
According to another feature of the invention, the composition is heated to a temperature in the first mixer which lies in a temperature range of 130xc2x0 C. to 180xc2x0 C. The advantage of this embodiment is that an accelerated reaction of the silicic acid with the silane takes place already in the first mixer. Furthermore, in this case, no further temperature increase of the composition is needed in the second mixer. A further advantage of this embodiment is that dwell times for the composition in the individual mixers can be matched optimally to each other.
According to another embodiment of the invention, the dwell time of the composition in the first mixer corresponds at least approximately to the dwell time of the composition in the second mixer. The advantage of this embodiment is that the two mixers can always be used simultaneously and can therefore be matched to each other so that a transfer of the composition from the first mixer into the second mixer and a discharge of a rubber base composition, which is mixed in the second mixer almost to the end, can take place simultaneously or only shortly one behind the other. In this way, an optimal usage of the capacity of the two mixers and the production of a rubber base composition in an optimal clock time is possible. This clock time corresponds to the dwell time of the composition in a mixer.
Another embodiment of the method of the invention includes the steps of: a first time duration is needed in order to uniformly distribute the composition ingredients; a second time duration longer than the first time duration is needed in order to bring about at least almost a complete reaction of the silicic acid with the silane; the dwell time in the first mixer and the dwell time in the second mixer corresponds to half of the total time composed of the first and second time durations; and, the temperature of the composition in the first mixer is held in a temperature range of 130xc2x0 C. to 180xc2x0 C. over at least a time span which amounts to the difference between the second time duration and the dwell time of the composition in the second mixer.
The advantage of this embodiment is that the dwell time of the composition in the mixers also matches when the reaction of the silicic acid with the silane takes a longer time duration than the dispersion of the composition ingredients to a composition (which is especially often the case for the rubber base compositions for producing motor vehicle tires).
According to another feature of the invention, the first mixer is a ram type mixer. The advantage of this embodiment is that a high pressure can be built up in the first mixer with the aid of the ram so that excellent dispersion of the composition ingredients is ensured even at the relatively low temperatures in the first mixer.
According to another embodiment of the invention, the rotors of the first mixer mutually engage. The advantage of this embodiment is that an improved and more rapid dispersion of the composition ingredients is possible than for a mixer without interengaging rotors because the ratio of the surface of the rotors to the fill volume of the mixer is improved. The more rapid thorough mixing of the composition ingredients in the first mixer leads to shortened mixture times for the rubber base composition.
According to still another embodiment of the invention, the second mixer is a ramless mixer. The advantage of this embodiment is that ramless mixers are open and, for this reason, a simple transfer is possible of the rubber base composition from the first mixer into the second mixer. A further advantage of this embodiment is that a simple and reliable removal of the reaction products of the silicic acid/silane reaction is possible from the open ramless mixer, so that a high quality rubber base composition is produced.
The rotors of the second mixer can be tangent to each other. In still another embodiment of the invention, the rotors of the second mixer, however, mutually engage. The advantage of this embodiment is that an interengaging mixer has an excellent ratio of the surfaces of the rotors to the fill volume of the mixer and thereby it is ensured that, within a relatively short time, the total composition runs through between the rotors of the second mixer and, as a consequence, a thorough mixing of the silicic acid with the silane results.
According to another embodiment of the invention, the second mixer has a greater fill volume than the first mixer. Preferably, the fill volume of the second mixer is greater than the fill volume of the first mixer by 20 to 60%. Use is made of this embodiment when the first mixer is a ram mixer and the second mixer is a ramless mixer. The advantage of this embodiment becomes understandable when the following is considered. In a ram mixer, the composition is pressed continuously by the ram between the rotors of the mixer so that an excellent through mixing of the composition is ensured. In contrast, in a ramless mixer, the problem is present that the composition leaves the region between the rotors because of the rotation thereof and, because of the non-present ram, cannot again come into this region. A thorough mixing of the composition is then no longer ensured. This problem is that much greater the smaller the fill volume is of the mixer. The advantage of the embodiment is therefore that, because of the greater fill volume of the second mixer, virtually the entire composition can be accommodated in the region between the rotors and is almost always disposed in this region so that an excellent through mixing of the rubber base composition in the second mixer is ensured even without the ram.
According to still another feature of the invention, a suction device is mounted between the mixers with which reaction products can be drawn off by suction. These reaction products arise during the reaction of the silicic acid with the silane in the second mixer. The advantage of this embodiment is that a rapid and complete removal of the reaction products is ensured whereby the mixing times in the second mixer are shortened and the quality of the rubber base composition is again increased.
According to another embodiment of the invention, the transfer of the composition from the first mixer into the second mixer takes place by utilizing the force of gravity. In this case, the first mixer is preferably mounted directly above the second mixer. An advantage of this embodiment is that no ancillary means (for example, in the form of a conveyor belt, et cetera) is needed. A further advantage of this embodiment is that the two mixers are arranged relative to each other so that space is saved.